In medical practice, medical instruments with a pointed needle are used very extensively, such as syringes and blood collectors. These medical instruments are used and treated by not only professionals in medical institutions but also non-professionals at home, e.g., a diabetes patient injects insulin on his own.
Such a medical instrument as a syringe may leave its needle to be exposed outside after use and likely stick relevant persons, which causes considerable difficulty to collection and treatment thereof. Such needle-stick might cause infection of bacteria.
In order to prevent occurrence of accidental needle-sticks, syringes with needles capable of being retracted after use have been proposed. However, such syringes are structurally complicated and costly.